1. Field of the Invention
This invention relates in general to filter devices and more particularly to a transmission line filter which can be incorporated within a main line transmission system, and also describes a method for making a particular embodiment of the transmission line filter.
2. Description of the Prior Art
In various types of high frequency systems, such as microwave systems and television systems, it is necessary to incorporate a filter within the system. Numerous filter devices are presently available. However, most of these filters are complex and difficult to fabricate. Those filters which are produced in large quantities at inexpensive and reduced cost have restrictive limitations and usually do not provide narrow bandwidth or deep nulls. To achieve such narrow bandwidth and deep nulls requires more complex filtering devices. Additionally, the filters generally used in the art require separate coupling to the transmission system and cannot be directly interconnected in series with the transmission line.
One specific area which finds great use and need for a transmission line filter is in connection with cable television. In cable TV systems the programs are sent out along a main cable and various taps are positioned along the main cable which interconnect the various subscriber lines. It is necessary, however, to be able to control the programming to each subscriber so that only the subscriber paying for a particular program will receive it. Each subscriber generally has a discrete address on his line to direct the program along that subscriber line. However, various means are needed to insure that only a paid-for program will be sent along a particular subscriber line. Security systems are therefore needed within the cable TV system. Maximum security systems involve scrambling the video along with the audio as well as the color carrier. However, such security systems add cost to the TV programs. At the other extreme are some cable TV systems which do not have any security and only rely on automatic polling ater the program has been on for a considerable length of time. An in between compromise is to utilize filters at the tap points of the feeder line. The cost and complexity of the filters, as well as the ability to controllably switch such filters is therefore an important part of a successful cable TV system.